The effect of mixidine fumarate on myocardial chronotropic responses to various stimulants was examined. Mixidine decreased elevated heart rate in the anesthetized dog to basal levels. It produced a dose-related decrease in heart rate elevated reflexly by aminophylline, by beta adrenergic stimulation induced by isoproterenol, by sympathetic nerve stimulation and by intravenous infusion of glucagon. Mixidine attenuated the increase in contractile force produced by sympathetic nerve stimulation but not that induced by isoproterenol. The compound antagonized the increase in rate of isolated guinea-pig atria induced by both isoproterenol and histamine. In the conscious dog, mixidine caused no decrease in resting heart rate, mean arterial pressure and cardiac output. It reduced atropine-induced sinus tachycardia as well as that induced by treadmill exercise. Experiments in the dog heart-lung preparation indicated that attenuation of an epinephrine-induced sinus tachycardia led to a decrease in myocardial oxygen consumption and an increase in myocardial efficiency. These studies suggest that mixidine fumarate induces an antichronotropic activity by a direct effect on the sinoatrial node and by attenuating sympathetic nervous system input to the heart.